1. Field of the Invention
The present invention relates to a drill for drilling in stone and, particularly, in concrete, and which is inserted in a mechanized hand-held tool and is subjected to rotational and percussion movements for cutting abrasive material.
2. Description of the Prior Art
The drills of the type described, because they are used for removal of abrasive material, require that the parts, which are inserted in the drill head, be made of hard metal. The insertable parts are fixedly secured in the drill head. Usually, the parts, which are formed as hard metal plates, are inserted in axially extending mortises formed in the drill head and are soldered to the drill head. Such a drill is disclosed, e.g., in German Publication DE 20 08 825. In the drill disclosed in DE 20 08 825, the bits smoothly pass into associated helix ridges of a helix used for removal of the drilled or cut material. To insure a more reliable securing of the hard metal plates in the drill head, according to European Publication EP 09 47 662 A1, the hard metal plates have additional prismatic projections which engage in corresponding recesses provided in the drill.
The drawback of such attachment of hard metal plates in axial mortises consists in that the hard metal plates easily break out upon a possible radial hooking of the places, in particular when the drill strikes a reinforcing metal. An encounter of the drill with reinforcing metal, because of a limited flexural strength of a hard metal plate due to its small thickness, results in generation of a localized tensile stresses in boundary surfaces of the mortises in which the portion of a respective plate engages. The localized tensile stresses of the boundary surfaces facilitate the breaking out of the hard metal plate. Also, the axially extending boundary surfaces are subjected to an excessive fatigue, which is caused by shear stresses resulting from the drill being subjected to axial percussions. A further drawback of the above-described attachment of hard metal plates consists in that the provision of mortises for receiving the hard metal plates reduces free space available for removal of the drilled-off material.
German Publication DE 43 39 245 discloses forming an entire drill head of a hard metal and securing the drill head to the drill body only over the end surface of the drill head which extends substantially perpendicular to the rotational axis of the drill. The solid hard metal head has a compact, substantially convex, polygonal shape with the lengths of its sides being substantially the same. Because of its shape, the solid hard metal head has a high inner stiffness and does not require provision of axially extending boundary surfaces which would facilitate breaking out of the bits. Rather, the boundary surface, which extends in the radial direction, is subjected to a compression pulsating load. However, the remaining superimposed torsional stresses can result in a torsional fatigue of the radial edge of the boundary layer. To prevent hooking in reinforcing metal, which leads to increased torsional stresses, usually, auxiliary cutting edges are provided between the main cutting edges and which are arranged exclusively on the radial edge of the head and are axially offset backward. The auxiliary cutting edges, due to their penetration resistance and mass inertia, prevent the main cutting edges from penetrating deeply into the reinforcing metal by temporarily lifting the drill from the reinforcing metal. The manufacturing and economical drawbacks of this drill result from a large volume of the solid hard metal head when the drill is designed for drilling large-diameter bores.
European publication EP 88 44 48 discloses a solid hard metal head with an xe2x80x9cXxe2x80x9d-shaped arrangement of four cutting edges, with the main cutting edges and the auxiliary cutting edges, which are formed as side cutting edges, being arranged, respectively, diametrically opposite each other and with respective main and auxiliary cutting edges forming an angle that deviates from 90xc2x0. The xe2x80x9cXxe2x80x9d-shaped, solid, hard metal head has essentially a rotationally symmetrical, crosswise, axially mirror symmetrical, prismatic shape. The base surface is provided with deep, concave, obtuse-angled notches which extend radially inward along one-third of the base surface parallel to the cutting edges over the longer surfaces. The notches serve for removal of the cut abrasive material. The side cutting edges trail the main cutting edges in the rotational direction, forming with the respective main cutting edges acute angles. As a result, a large recess on each of the short sides of the base surface is limited by the radial edge region and serves for removal of some of the cut abrasive material in front of the respective auxiliary or side cutting edge. Because in EP 88 44 48, the head is secured to the drill body only over its end surface, torsional stresses, which are generated upon hooking of the drill with the reinforcing metal, result in the torsional fatigue of the head end surface. Another drawback of this head consists in that because of an obtuse angle, in the rotational direction, between the auxiliary cutting edge and trailing the auxiliary cutting edge, the main cutting edge, a large rotational angle is needed in order to overcome the mass inertia. However, practically, it is impossible to form such an angle in low-speed, large-diameter drills.
German Publication DE 197 07115 discloses a drill with an even number, but more than two, of radial, arranged rotationally symmetrical, bits and having a prismatic, solid, hard metal head, with the bits not being formed as separates plates but forming an integral part of the head. The head is secured in corresponding grooves having axially extending surfaces and formed in the drill body. The head is secured to the drill body over the head end surface and along the surfaces of the grooves. The securing the head over its end surface and along the surfaces of corresponding mortises insures, even in large diameter drills, break-proof securing of the bits, and the danger of the bits breaking out is substantially reduced even if the drill encounters a reinforcing metal. However, the drawback of this drill consists in that providing axial mortises on opposite sides of each bit reduces free space for forming respective recesses necessary for removal of the drilled-off abrasive material.
Accordingly, an object of the present invention to provide a rock drill without the drawbacks of conventional prior art rock drill.
Another object of the present invention is to provide a rock drill with break-proof securing of the hard metal bits and which would provide a sufficiently large space for removal of the drilled or cut-off abrasive material, while insuring a high drilling efficiency.
This and other objects of the present invention are achieved by providing a rock drill in which the solid hard metal head is secured to the drill body over the end surface of the head and is additionally secured to the drill body over portions of the side surfaces associated with respective bits, located in a radially outer region of the head, extending substantially parallel to a rotational axis of the drill, and extending into mortises formed in the drill body, with the side surfaces including first surfaces, which are associated with at least some of the bits and are subjected to pressure acting in a rotational direction of the drill during operation of the drill.
According to the present invention, the solid hard metal head in addition to being secured to the end surface of the drill body, which lies perpendicular to the rotational axis, is secured to the drill body along a portion of the side surfaces of the bits which lie in the outer radial region of the drill head and extend, advantageously, parallel to the rotational axis. The side surfaces of at least some of the bits support the respective bits in the rotational direction, i.e., they are subjected to a certain pressure. As a result, a break-proof attachment of the solid hard metal head to the drill body is achieved even at high torsional loads applied to the drill.
Advantageously, other portions of the bit side surfaces, which are not used for securing the solid hard metal head to the drill body, form a free transition to a recess that serves for removal of the drilled-off abrasive material. The material discharge recess lies in front of a respective main bit, thereby direct removal of the cut-off material, i.e., without a substantial carrying of the removable material in the rotational direction, takes place.
In the drills with an even number of bits which, preferably, divided in main and auxiliary bits, and which are arranged rotationally symmetrically, it is, advantageously, the main bits that are supported in the rotational direction. Thus, the side surfaces of these bits, which face in the rotational direction, remain free. The recess, which is associated with a respective main bit and lies in front of the respective main bit in the rotational direction is limited, advantageously, by a free, facing in a direction opposite to the rotational, side surface of an auxiliary bit that lies in front of the respective main bit in the rotational direction. In this way, as large as possible recess is provided in front of the respective main bit. The adjacent side surfaces of the adjacent main and associated therewith, auxiliary bits, which take part in securing of the drill head to the drill body and inner ends of which angularly butt-join each other, form a smaller recess in front of, in the rotational direction, an auxiliary bit, but the size of which is sufficient for removal of smaller amount of material cut off by the auxiliary bit. In this way, even with a high drilling efficiency, removal of the cut-off or drilled-off material is insured.
In a drill with four bits, two main bits and two auxiliary bits, which arranged, respectively, diametrically opposite each other, angularly adjoining each other, side surfaces of the adjacent main and auxiliary bits, which extend parallel to the rotational axis, are used for an additional securing of the drill head to the drill body. The remaining side surfaces of the main and auxiliary bits, which are likewise angularly adjoin each other in pairs and extend parallel to the rotational axis, remain free. Thus, with a solid hard metal head having four, substantially radially extending bits, each two, arranged substantially diametrically opposite each other, respective side surfaces are used for securing the drill head with the drill body, with each two other, likewise arranged diametrically opposite each other, side surfaces, which remain free, forming the larger recesses for removing the abrasive cut-off material.
For drills having main and auxiliary bits, it is advantageous when an auxiliary bit is located, in the rotational direction of the drill, in front of the associated therewith, main drill as this permits to reduce a torsional load, in particular when the drill encounters a reinforcing metal. The reduction of the rotational angle, which has to be bridged to overcome the mass inertia and, thereby, the reduction of the penetration time at the given rotational speed permit to reduce penetration of the main bit into the reinforcing metal and, thereby, to reduce torsional stresses in the bit. A relative diminishing of the recess in front of the main bit, which is caused by locating the associated auxiliary bit in front of it, in the rotational direction, is compensated by a relative increase of the free-lying side surfaces of the auxiliary bits. Thereby, it is insured that the same amount of the drilled-off material can be removed, while a smaller penetration of the main bit into the reinforcing metal is achieved.
An advantageously vertical, not radial, arrangement of the free surfaces of an auxiliary bits trailing respective radially extending main bits increases the inner rigidity of the solid hard metal head and improves the removal of the drilled-off abrasive material by reducing the adjoining frictionally engageable side surfaces.
Advantageously, the cross-sectional surface of the drill head, which is necessary to provide for securing of the drill head to the drill body and to insure that the respective side surfaces extend into the mortises, smoothly passes into the drill body, with the outer radial regions of the drill head passing into the corresponding helix ridges, and with the main and auxiliary bits passing into the corresponding main and auxiliary helix ridges. Thereby, transmission of forces into the drill body and removal of the drilled-off material is insured. The foregoing structure also insures that the separate recesses for the respective main and auxiliary bits and combined in a common large helix ridge with small frictionally engageable surfaces.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings.